For those who have trouble keeping the two apart and actually care about speaking correctly here is a short article about how to tell when to use “who” and when to use “whom.”
“Who” refers to the subject of a clause and “whom” refers to the object of a clause. There is an article here that does a great job of explaining the difference. It does a better job than I ever could so for now I will stick to explaining the quick and dirty method of determining which to use.
If you form your sentence into a question and the answer is “he/she,” you would use “who.” If the answer is “him/her,” you would use “whom.”
Examples:
* Who/whom wrote the book?
He wrote the book, therefore “who” is correct.
* Who/whom is the best?
He is the best, therefore “who” is correct.
*Who/whom do you love”
I love him, therefore “whom” is correct